As U.S. Attorney General Jeff Sessions told a crowd of federal, state and local law enforcement in March, the country "is in the throes of a heroin and opioid epidemic." According to estimates from the Centers for Disease Control and Prevention, prescription opioid and heroin overdoses kill 91 Americans each day.

But in the same speech, Sessions made clear that he thinks the drug crisis isn't limited to opiate abuse. "I am astonished to hear people suggest that we can solve our heroin crisis by legalizing marijuana — so, people can trade one life-wrecking dependency for another that's only slightly less awful," he said.

Studies have shown a different link between marijuana and opioids, however. "Really, if we stopped medical marijuana programs that are now in place in 29 states and Washington, D.C. … the science suggests we would worsen the opioid epidemic," says Dina Fine Maron, a medicine and health editor at Scientific American, who wrote a recent story on the subject.

She explains that states with medical marijuana programs have fewer opioid overdose-related deaths than states without medical marijuana — 25 percent fewer, according to a 2014 study cited in her article.

"The reality is that the literature right now suggests that if anyone is using an opioid — whether it be a prescription painkiller or something like heroin — a prescription painkiller is more likely [than marijuana] to lead to drug abuse," she says, "because it's more addictive and obviously can be more lethal."

According to research from the National Institute on Drug Abuse, Maron explains, the majority of people who use marijuana do not go on to use harder drugs. Meanwhile, nearly half of young people who inject heroin first abused prescription opioids. University of Georgia public policy professor W. David Bradford has studied how legal medical marijuana impacts prescription use by enrollees of Medicare, the federal health insurance program for seniors and the disabled. "What we found … was significant reductions in prescription use, most notably among pain medications, and the largest plurality of those would be opiates," he says.

Then he researched the effect on enrollees in Medicaid, the federal-state program that helps the poor and people with disabilities pay for health care. "We redid the study for Medicaid just this past month in Health Affairs and, again, found large reductions in the use of prescription pain medications when states turned on medical cannabis laws."

But in a letter to Congress in May, Attorney General Sessions asked lawmakers to allow a key medical marijuana amendment to expire, citing the "historical drug epidemic." Currently, the amendment keeps the Department of Justice from using taxpayer dollars to interfere with state medical marijuana programs, Maron says. "And if it's not renewed, that means that Jeff Sessions and his office could go ahead and prosecute dispensaries."

Legal medical marijuana isn't a silver bullet for the complex U.S. opiate crisis, Bradford says. But while dozens of people in the U.S. die each day from opioids, there has never been a fatal overdose documented from marijuana alone.

"The National [Academies] of Sciences, Engineering, and Medicine just this past January issued a comprehensive report where they said there is conclusive evidence that cannabis can be effective at managing pain," he says.

"So, to the extent we can divert people from initially starting on opiates through legitimate prescriptions, we divert them from the path of abuse and then the path of death," he adds. "And it does seem that cannabis could be one tool in the arsenal to do that."